Test Perfect

Jan Burse, created Sep 15. 2018
* Prolog code for finding a perfect number.
* A solution that did not make use of findall/3 and rem/2
* was posted Dec 16, 1988 on comp.lang.prolog by Thomas
* Sj|land. It was titled Christmas pleasure and contained
* an invitation to further dig into Euclids theorem.
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% between1(+Integer, +Integer, -Integer)
between1(Lo, Hi, _) :-
Lo > Hi, !, fail.
between1(Lo, _, Lo).
between1(Lo, Hi, X) :-
Lo2 is Lo+1,
between1(Lo2, Hi, X).
% sumlist1(+List, -Integer)
sumlist1([], 0).
sumlist1([X|Y], R) :-
sumlist1(Y, H),
R is X+H.
% perfect(+Integer, -Integer)
perfect(Hi, X) :-
between1(1, Hi, X),
Y is X//2,
findall(Z, ( between1(1, Y, Z),
X rem Z =:= 0), L),
sumlist1(L, X).
% perfect
perfect(500, _).